SUMMARY.Wastwater (English Lake District) was sampled at monthly intervals over 1 year to compare the seasonal and vertical extents of floristic and physiological variation in an oligotrophic phytoplankton assemblage. Cellular photochemical capacity (CPC) was measured byin vivochlorophyll fluorescence before and after addition of the photosynthetic inhibitor 3(3,4‐dichlorophenyl)‐1,1‐dimethyl urea (DCMU). The CPC index proved useful to define an activity coefficient for the chlorophyll levels measured in boiling methanol extracts. Large variations in CPC over time or depth were generally correlated with major shifts in phytoplankton species composition. Highest CPC values were recorded during autumnal mixing. Both CPC and floristic composition of the community demonstrated as great a variation over depth during a period of stratification as that recorded over time throughout the year. These observations emphasize the multi‐canopy structure of the Wastwater phytoplankton community in